The Ghawar Oil Field, Saudi Arabia

The Ghawar Oil Field is by far the largest conventional oil field in the world and accounts for more than half of the cumulative oil production of Saudi Arabia. Although it is a single field, it is divided into six areas. From north to south, they are Fazran, Ain Dar, Shedgum, Uthmaniyah, Haradh and Hawiyah. Although Arab-C, Hanifa and Fadhili reservoirs are also present in parts of the field, the Arab-D reservoir accounts for nearly all of the reserves and production.

The Ghawar Field was discovered in 1948. Production began in 1951 and reached a peak of 5.7 million barrels per day in 1981. This is the highest sustained oil production rate achieved by any single oil field in world history. At the time that this record was achieved, the southern areas of Hawiyah and Haradh had not yet been fully developed. Production was restrained after 1981 for market reasons, but Ghawar remained the most important oil field in the world. The production of the Samotlor Field in Russia was greater during the mid-eighties, but this was because production at Ghawar was restrained. Development of the southern Hawiyah and Haradh areas during 1994 to 1996 allowed production from the Ghawar Field to exceed 5 million barrels per day once again, more than Samotlor ever produced.

This remarkable production history is because of the enormous size of the Arab-D reservoir in the Ghawar Field. Alsharhan and Kendall (1986, Table 1) provide a figure of 693,000 acres for the productive area of the Ghawar Field. This represents a single, pressure-continuous reservoir. Cumulative production by yearend 2000 was about 51 billion barrels of oil.

The anhydrite in the Upper Arab-D forms the seal for the 1,300-foot oil column in Ghawar. It is composed of sabkha evaporites and subaqueous evaporites with thin carbonate interbeds that can be traced for hundreds of kilometers. The anhydrite thickens to the south at the expense of the reservoir zones; the combined thickness remains relatively constant.

The Arab-D reservoir at Ghawar comprises two major shoaling-upward cycles deposited during a relative highstand in sea level (Mitchell et al, 1988). It is composed of skeletal grainstones and packstones with ooid grainstones locally common in the upper Arab-D. The diagenetic processes that have affected the Arab-D reservoir include dolomitization, leaching and recrystallization, cementation, compaction and fracturing.

Interparticle porosity is abundant in the Arab-D reservoir in the Ghawar Field and moldic porosity is common as well. Intercrystal pores are common in dolomites and microporosity is abundant in both limestone and dolomite lithologies.

In the uppermost part of the Arab-D are occasional zones that contain more than 10% of a stromatoporoid sponge known as cladocoropsis. Where this facies is dolomitized, the relatively fine-grained matrix is replaced by dolomite and the cladocoropsis is leached, causing a phenomenon described by reservoir engineers as super-permeability. These super-permeable zones, where present, offer so little resistance to fluid flow as to be difficult to model for reservoir engineering purposes.

The source rock for the Ghawar oil is believed to be the Tuwaiq Mountain Formation, which underlies the Hanifa. It is Callovian and Oxfordian in age and reaches a thickness of more than 300 feet in the basinal area between the Ghawar and Khurais Fields. That this moderate volume of source rock should produce the largest accumulation of light oil in the world indicates very efficient migration and entrapment. The fact that the Ghawar oil-water contact is substantially higher on the west flank than on the east indicates a hydrodynamic gradient to the east, which may explain the much larger volume of oil in Ghawar than in Khurais.

The Ghawar structure consists of two subparallel, north-south trending structural crests, separated by a saddle. It is about 174 miles long and 12 miles wide. In the northern part of the field, the saddle dips below the initial oil-water contact between Uthmaniya and Ain Dar, but the two crests remain above it. The Fazran and Ain Dar areas are along the western crest and the Shedgum area is along the eastern crest. The three southern areas extend across both crests. These crests existed at the time of Arab-D deposition and the reservoir quality is best in the crestal areas. A map of the Ghawar structure at Arab-D level appears below.

The oil-water contact at Ghawar dips to the northeast, dipping more than 660 feet between the southwest end of Haradh and the Fazran area. The contact is consistently higher on the west flank of the field than on the east, and a tar mat is associated with the original contact. Water injection wells are completed above this tar mat for pressure maintenance.

This peripheral waterflood project began in the early sixties in the northern parts of the field. By the time that the southern Hawiyah and Haradh areas were developed during 1994 to 1996, horizontal-drilling technology was available. Horizontal injectors were completed above the tar mat to provide line source distribution of water along the periphery of the field.